The recordable compact disc (CD-R), on which information can be recorded only once, is widely used, for instance, as a large capacity computer data disc.
The optical disc of CD-R type generally comprises a disc-shaped substrate (support) and a recording layer provided thereon. The disc substrate comprises a transparent material such as synthetic resin. The recording layer comprises a metal or a semi-metal such as Bi, Sn, In or Te, or alternatively a dye such as a cyanine dye, a metal-complex dye, or a quinone dye. A light-reflecting layer and a protective layer may be superposed in order on the recording layer. The light-reflecting layer and the protective layer can be made of gold and resin, respectively.
On the CD-R, the writing (recording) and reading (reproducing) can be performed in the following manner. The laser beam (usually used wavelength is 780 nm) modulated by information is applied to the recording layer of the CD-R through the substrate. The recording layer alters its optical characteristics in the area where the laser beam has been applied, to produce a chemical or physical change such as production of pit. The reading of the recorded information can be performed by sequentially applying a laser beam on the recording layer of the CD-R through the substrate and detecting a light reflected on the CD-R.
At present, most of the recording layer of CD-R utilizes a light-sensitive dye as the recording material, because the recording dye layer can be easily formed by a coating method and shows a high sensitivity, as compared with the recording metal layer. However, the dye layer has such disadvantages as low light-resistance and low heat resistance. Therefore, it has been desired to develop a recording layer having high durability against light and heat, as well as having high recording and reproducing characteristics.
Japanese Patent Provisional Publication No. 63-209995 describes an optical disc which has a recording layer comprising an oxonol dye. According to the description of publication, such optical disc maintains stable recording and reproducing characteristics for a long time. The publication discloses an oxonol dye compound having an ammonium ion in the form of an inner salt.
The inventors have found that the optical disc having the recording layer of the above oxonol dye exhibits insufficient light-resistance, while the disc has relatively improved characteristics in view of heat resistance and durability for repeated reproducing. Therefore, such optical disc often causes troubles in reproducing process after long exposure to light such as sunlight.
Accordingly, it is an object of the present invention to provide an information recording medium having stable recording characteristics and particularly high light-resistance to maintain satisfactory recording characteristics for a long time.
It is another object of the present invention to provide a new oxonol dye compound favorably employable for preparing the information recording medium having stable recording characteristics.